1. Field of the Invention
This invention relates generally to a temperature control apparatus and method for active control of a Stirling-cycle cryocooler cold finger tip temperature, by adjusting the cryocooler compressor piston stroke amplitude, and more particularly, to a temperature control apparatus and method with a PID (proportional/integral/derivative) control law that uses proportional, derivative, and integrated temperature error information to generate the required compressor piston stroke amplitude change for achieving the precision temperature control, followed by a distribution law that distributes the desired piston stoke amplitude change to each compressor motor so that the force balance at the fundamental frequency is maintained.
2. Description of the Related Prior Art
A primary application for cryocoolers is with superconductive electronic devices and electro-optical devices, operated at cryogenic temperatures. Due to the fact that their electrical resistance is very low and they are extremely sensitive to magnetic radiation, superconductor electronic devices must be operated at very low temperatures and in low magnetic fields.
Many refrigeration systems utilizing Stirling-cycle apparati have been developed for cryogenic cooling. Representative of the U.S. Patents relating to cryogenic cooler apparati are U.S. Pat. Nos. 5,245,830; 5,146,750; 5,032,72; 4,606,194; 4,429,732; 4,413,473; 4,413,474; 4,413,475; 4,078,389. In general, these apparati may be described as systems where the fluid, such as helium, is initially cooled by passing through a regenerator, while maintaining high pressure, and then finally cooling the initially cooled fluid through expansion and discharge.
The performance of many detector devices, used for detection or measurement of very small incident signals, is enhanced by reducing the temperature. Infra-red detectors and similar heat-sensitive instruments have dramatically increased the need for cryogenic cooler apparati, since without cooling, the infra-red (IR) signal of distant IR sources will be masked by heat energy. Thus, cryogenic coolers, capable of continuously maintaining the required cryogenic temperature, are an essential component of space-borne infra-red (IR) surveillance sensors. In order to maintain optimal signal to noise ratio, these detectors have to be cooled to cryogenic temperature.
To optimize their IR sensitivity performance, the temperature at the IR focal plane array needs to be precisely controlled. Controlling the focal plane array temperature using an additional active thermal device, such as a heater, seems to be a simple solution. However, this approach requires additional hardware and electrical power which may not be desirable for space applications with stringent weight and power constraints. Another major problem in applying the cryocoolers to sensitive focal-plane instruments is the vibration induced by the reciprocating motion of internal components in both the cryocooler compressor and expander. If unbalanced, the motors in compressor and expander produce large forces which, in nearly all cases, are excessive for spacecraft and sensors.
Moreover, cryocoolers must be able to efficiently control space-borne IR focal plane arrays without creating vibration disturbances that would impair sensor performance. Therefore, the performance of the cryocoolers are dictated by thermodynamics and vibration consideration.
The pistons of the Stirling cryocoolers are typically powered by linear, electric drive motors which drive the pistons in reciprocation. The rate at which heat is pumped by the Stirling cooler, and thus the temperature of the system, is a continuous function of the piston displacement. Therefore, it is desirable to control the piston displacement as a function of temperature of the refrigerated cold finger tip, in order to stabilize the temperature within specific limits.
The control of the piston displacement is accomplished by controlling the drive signal applied to the cryocooler motor via feedback control loop. Conventional analog feedback controls, designed to reduce and/or to eliminate the temperature error, feed an analog error signal into a control loop. However, since the analog feedback control loop may not function satisfactorily, digital feedback control circuit utilization became necessary and possible with the appearance of the digital computers.
The approach of adjusting the compressor piston stroke amplitude to control the cold finger tip temperature had been previously investigated by Robert R. Clappier and Robert J. Kline-Schoder, of the Lockheed Missiles & Space Company, Inc., and described in their paper "Precision Temperature Control of Stirling-cycle Cryocoolers," 1993 Cryogenic Engineering Conference, Albuquerque, N.M., Jul. 12-16, 1993. However, the temperature control algorithm developed by Lockheed does not deal with the fundamental frequency vibration control which is one of the critical requirements for a high performance space-borne IR sensor.
Another approach of adjusting the compressor piston stroke amplitude to control the cold finger tip temperature had been previously investigated by J. N. Aubrun, R. R. Clappier et al. and described in their paper "A High-Performance Force Cancellation Control System for Linear Drive Split-Cycle Stirling Cryocoolers," CEC Conference Proceedings (11-14 Jun. 1991). However, this method uses two back-to-back compressors with a hybrid digital/analog control system where the pistons' movements are modulated proportionally, by following identical waveforms. It does not provide a distribution law module that distributes the desired piston stroke amplitude change to each compressor motor so that the force balance at the fundamental frequency is maintained.
U.S. Pat. No. 5,245,830, issued to Aubrun, illustrates an adaptive control for reducing control system error to near zero, by feeding forward in time a correction, in order to deal with a problem before it happens. The invention smooths, by a local averaging process, the effect of random noise. The adaptive control system measures, during a cycle of operation, the error between a desired cycle command and an output signal representing actual system operation. The smoothed, time shifted, error correction data is converted and combined with the desired cycle command to produce an adapted desired cycle command for reducing control system error to near zero. However, this method does not simultaneously control the cold finger tip temperature and balance the fundamental frequency vibration forces.